Volume 8 Issue 2 (2020) 82-85 ISSN 2347 - 3258 International Journal of Advance Research and Innovation Review on Sheet Metal Formability Shivang Chhikara, Rajat Khatta, Suraj Prakash Verma, Dadge Mukesh Shamrao Department of Mechanical Engineering, Delhi Technological University, Bawana Road, New Delhi-110042, India Abstract: Sheet metal Forming is one of the most important and diverse manufacturing processes Article Info and has been an integral part of industries like automotive and aerospace which need high density, Article history: lightweight and high strength materials. This paper tries to encapsulate the features related to Received 17 January 2020 Formability and its testing. Various parameters like anisotropy and strain rates on which the Received in revised form formability depends are also considered and their impact on the testing. Further, the paper 20 May 2020 Accepted 28 May 2020 includes study of Formability of Tailor Welded Blanks which is an integral part of the automotive Available online 15 June 2020 sector. At last the paper concludes with the future developments that can be made in this field Keywords: Formability, Forming Limit along with new modelling techniques and new advancements in testing. Diagram, Yielding, Anisotropy, Strain Rate

1. Introduction stretching is measured and the metals are rated on the basis of them. Sheet-Metal Formability is generally defined as the ability of the Formability tests are mainly divided into three broad categories- material (typically called as blank) to undergo plastic deformation intrinsic tests, simulative tests and the test that are used to plot the governed by flow rules to give the desired shape change without FLDs. The intrinsic tests consist of basically the uniaxial tension tests and are independent of the sheet metal geometry. In simulative tests, failure, such as wrinkling, cracking, necking, cracking, tearing, etc. drawing and stretching conditions are simulated because these are the Formability is not easily quantified as it depends on several factors most common ones in the industrial applications. The different types such as material flow properties, ductility, die geometry, die material, of simulative tests are Erichsen Olsen Tests, Swift Cup test, Limit lubrication, press speed, stress rate, strain rate, temperature Dome Height test and OSU tests. There are two experimental roughness, springback, strain localization, etc. [1]. Further, methods for obtaining FLDs, one is, Out-of plane formability tests formability of different materials also depends upon the sheet metal [5,6] and the second one is, In-plane formability tests [7,8]. For both process that is being undertaken for the production of the part. the tests, a grid marking scheme is used in which the sheet metal is Important processes in this field are bending, deep-drawing, stretch marked with a grid pattern and is deformed in the stretching tests [9]. forming, hydroforming, incremental forming and many more. The After this the deformation of the grid pattern is measured where either formability of any sheet is at its minimum at plane strain conditions necking has taken place or fracture has occurred, thus, giving the and so about 80-85% industrial failures occur around this state. So, values of major and minor strains. Examples of these tests include an ideal test should be able to simulate these conditions efficiently Nakajima Test [10], Marciniak Test [11]. and effectively. It should also be taken into consideration that no single formability test can relate the formability for all types of forming or stamping operations [2]. The basic forming characteristics of sheet metals can be obtained from simple mechanical tests such as tensile, bulge and hardness tests but these tests are too simple to relate to various parameters and phenomena such as strain-hardening. A few important parameters on which the formability of a sheet metal depends on are the strain-hardening index (n) which is usually obtained from flow curves (fig.1), strain rate and anisotropy of the material. Higher the strain hardening index and higher the strain-rate index(m), higher is the material’s ability to undergo large uniform strain before necking. Further, improvements in the drawability of the material is seen when the Anisotropy index (r) is higher. But there is a major disadvantage of these simple tests, i.e., they completely ignore the effect of variables (geometry of punch or die, lubrication, Fig.1: True stress versus True Strain Diagram [3] punch speed etc.) on the forming behavior of sheet metal completely. According to the power-law of strain hardening- Various theoretical models are also proposed to calculate forming limit diagrams of different sheet metals. These Modelling techniques 휎 = 푘휀푛 (1) are mostly based Bifurcation theory, geometrical imperfection theory and Continuum damage mechanics [4]. The modelling is also an 2. Formability Tests integral part of formability analysis because during the process of All the sheet metal industries have been looking for an ideal sheet metal forming, the plastic deformation itself includes a lot of formability test which can evaluate the metal’s ability to resist different phenomena like anisotropy, hardening, failure and fracture splitting failure that usually occur in plane-strain condition [1]. The that too occurring simultaneously. Thus, modelling or numerical most accepted way to predict of as sheet metal is by plotting the analysis tries to combine all these factors so that the cumulative effect Forming Limit Diagram (FLD) (fig.2). It is an experimental on formability can be seen. Some of the important modelling procedure in which a combination of two principal surface strains techniques developed are Hills criterion, Swift’s Model (Maximum (major and minor) are represented. The diagram consists of force criterion), Modified maximum force criterion (MMFC), Storen Formability Limit Curve (FLC) above which localized instability is and Rice (S-R) theory, Marciniak and Kuczynski (M-K) model and observed. The major and minor strains plotted in the FLC are one of the recent developments in this field is NADDRG model. obtained from these tests that are done under various train paths from Swift cup test evaluates the drawability of a sheet metal by simulating uniaxial tension over plane strain tension to biaxial tension [4]. In the drawing operation, in which the sheet is held under a blank holder formability tests the metal’s resistance to necking instability during and is drawn into small parallel sided cup. Drawability is measured by drawing various blanks with increasing the diameter one by one *Corresponding Author, and thus defines the Limit Draw Ratio (LDR) which gives an E-mail address: [email protected] estimation of the drawability. Swift cup test is highly standardized, All rights reserved: http://www.ijari.org

IJARI 82 Volume 8 Issue 2 (2020) 82-85 ISSN 2347-3258 International Journal of Advance Research and Innovation with die and punch radii as 6.35mm. But its drawback is that it is a 3. Anisotropy and Formability very time-consuming process Till the 1980s the yield criterion for materials did not have the

푀푎푥푖푚푢푚 퐵푙푎푛푘 퐷푖푎푚푒푡푒푟 required anisotropic coefficients that these criterions were not much 퐿퐷푅 = (2) 퐶푢푝 퐷푖푎푚푒푡푒푟 accurate. Hills provided the useful yield criteria for anisotropic materials in 1948 and it was the only one to have some applicability in a number of stress cases. The isotropic yield conditions for various metals were given by Tresca and Von Mises using stress (σi) and deviatoric stress (Sj) tensors. 푎 푎 푎 휙 = |휎1 − 휎2| + |휎2 − 휎3| + |휎3 − 휎1| = |푆1 − 푎 푎 푎 2 푆2| + |푆2 − 푆3| + |푆3 − 푆1| = 2휎 (3)

Here, σ1, σ2, σ3 are the principal stresses, 휎 is the effective stress. For a=2 and a=4 the equation becomes the Von mises criterion for a=1 and for the limiting case of 푎 → ∞ the equation becomes Tresca’s criterion. [15] For isotropic materials the yield criterions are independent of the reference frames. But for anisotropic materials yield properties are directional and thus the yielding criterion is independent on the reference frame. Thus, Hill extended the Von Mises criteria [16] to orthotropic materials (which exhibit orthotropic symmetry) 휙 = 2 2 2 2 퐹 (휎푦푦 − 휎푧푧 ) + 퐺 (휎푧푧 − 휎푥푥 ) + 퐻(휎푥푥 − 휎푦푦 ) + 2(퐿휎 푦푧 + 2 2 2 푀휎 푧푥 + 푁휎 푥푦) = 휎 (4) Here, F, L, G, H, M, N are material-based constants. This yield criteria provided accounts for planar anisotropy which is obtained Fig.2: Forming Limit Diagram (FLD) [12] from the equation (5) Erichsen and Olsen are basically two tests that simulate the 푟 +2.푟 + 푟 stretching operation conditions. In these tests, a sheet metal is 푟 = 0 45 90 (5) 0 4 clamped between two plates and a hole of diameter 25.4 mm is made These values of r are determined by uniaxial tension tests and then into it. Now, a ball with diameter, d, is pressed into the sheet until the cutting the sheet metal along three directions in the plane of the sheet fracture occurs. For Olsen test, d=22.2 mm and for Erichsen test, at 0°, 45° and 90°. d=20mm. Height of the cup, h at failure is used as formability index, Lankford et. Al [17] introduced the term strain ratio, or as it is higher this index is higher will be the formability of the material. The commonly called as the plastic anisotropy r-value and used this to disadvantages of this test are first they cannot be correlated with other represent the drawability of different sheet metals.it is also seen in mechanical properties of the sheet metal and second these tests have some researches that the limiting draw ratio in simulative tests is quite poor data reproducibility. Limiting Dome Height Test (LDH) closely related to the average r-value. Thus r-value of anisotropy has simulates the stamping conditions more effectively. This test includes been quite extensively used as a parameter for the drawability of sheet clamping the metal sheets of varying width into a blank holder and a metals [17]. 102 m hemispherical punch is used to stretch the sheet over. Small 4. Formability and Strain Rates diameter circles (2.5mm) are marked like a grid on the sheet and the Hecker [18] proposed that strain-hardening index (eqn. 6) was not circle closest to the fracture gives the width strain. The height at alone responsible for the prediction of fracture height in Olsen test which dome fails gives us Limiting Dome Hight (LDH) near plane and thus strain-rate sensitivity index also has to be taken into strain and thus is used as a formability index. The standard width used consideration. Thus, it was seen that for a given strain-hardening in the industry is 133mm. The disadvantages of this test are that it index (n), higher the strain-rate sensitivity index (m), the material will gives scattered results on various machines and tooling. Further, yield a larger cup height, therefore, indicating an increase in stable plane strain conditions are not achieved in this test due to the formability of the sheet metal [19]. Hence, for a given value of n steel axisymmetric tooling. In OSU (Ohio State University) test there is no has more formability than aluminum since it has higher strain-rate axisymmetric tooling of punch diameter of 25.4 mm thus giving sensitivity index stable plane strain conditions. In this test, 5 sheets are used of width 124mm and length 178mm and results are averaged. This test also 휕푙푛푙푛 휎 푚 = (6) defines formability index at the height at which failure occurs. OSU 휕휀̇ test is said to be 6 times faster than the LDH test. It is seen that formability increases with high speed due to the reasons such as constitutive behavior, inertial effect and die impact effect. Also, some materials show increase in ductility and low stress too. Strain rate effects also play a role in increased formability due to high-velocity [20]. Due to inertial aid, there is a change observed in the necking region which causes the increase in formability of sheet metal [21]. Dariani et al. [20] conducted a study on FLDs of Al 6066-76 & AISI 1045 sheets at various deformation velocities int the explosive forming process and found that formability increased by 53.8% by impact and 146.1% by explosive forming. Gerdooei & Dariani [22] analyzed the formability of metal sheet at varying strains (from 0.001 to 100/s) and their effects were shown on the critical plane strain (FLD0). Fig.3: Sketch of the limit dome height test on the left [13], and the sketch of swift cup test on the right [14]

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Volume 8 Issue 2 (2020) 82-85 ISSN 2347-3258 International Journal of Advance Research and Innovation Some non-conventional testing methods are also emerging nowadays such as Continuous Bending under Tension (CBT) test which generates cyclic stretch bending [28]. Tube Extension tests are also been put to use for the study of formability of sheet metals. Also tube forming is nowadays used in automotive radiator closure also and the advantage lies in the fact that this process has fewer components than the conventional process and stiffness of the parts are also significantly increased. Hydroforming is also one of the new techniques used for tubular products and enables testing under bi- axial stress state. Further techniques such as Incremental Forming are also gaining momentum because this process uses universal tooling and thus increases the flexibility of the forming method. Further, materials at very high temperatures are also formed by various sheet metal processes and the formability calculated under such circumstances is of utmost importance. So, formability analysis of such new techniques is also needed to ensure that there is reduction in costs and low scrap is generated and the forming methods could be used to different set of materials. So, at present the main focus of researchers and engineers should be on developing a more sophisticated testing equipment so that a single test ensures procurement of maximum possible information of formability of the Fig. 4: FLDs of Al 6061-T6 sheet under various strain rates sheet metal. Also, new modelling techniques and numerical analyses should also be developed which strengthen the theoretical concept of 5. Formability in Tailor Welded Blanks FLDs and can confine a lot more forming parameters and their As we are seeing, there is a worldwide trend of trying and achieving consequent effects. Further, advancements in the field of new low weight, high density, high strength materials with high materials should also be looked forward to, as the new materials open formability in industries such as automotive and electrical new horizons of high strength and lightweight application and thus applications. Therefore, the use of Tailor Welded Blanks (TWBs) can be beneficial to the industry. And this can further increase the have increased tremendously as this process allows materials of profits of the organizations. varying thickness, properties or surface conditions in a single pressed 7. Conclusions or stamped part [23]. The parts produced by this process have joints This paper tries to give an insight to the reader about the importance of high strength and high reliability. In making Tailor Welded of sheet metal forming methods and how formability analysis is an Blanks, 99.9% of the times either CO2 or Nd: YAG laser welding is integral aspect of it. The paper gives a brief overview of the various used [24]. But theses welding processes affect the formability of the tests involved in getting quantifiable data on formability and to get sheet metal or the end product too. Mustafa A. Ahmetoglu et. Al [25] the forming limits above which the failure occurs. There has been a concluded in their study that in case of TWBs failure in deep drawing lot of advancements in formability over the past four decades and still usually occurs at the flat bottom of the punch parallel to the weld line the research continues in search of new materials, comprehensive and this happens due to the non-uniform distribution of the testing methods & equipment and mathematical analyses of the deformation. According to Hisashi Kusuda et. al. [26], in automotive yielding criterion considering a lot more parameter and their effects sector, TWBs are used in making three different parts- 1. Side panel, on formability. So, this paper has been an attempt to study the basic outer 2. Door, inner 3. Front side inner and the formability problems nature of formability and its importance in the industry of today. seen in these three respectively are split in stretch flanging, stress References concentration in thin plate and insufficient ductility of weld bead. [1]. K. Narsimhan, M P Miles, R.H. Wagoner, A better sheet- Thus, by considering these 2 studies, a point is very clear that there is formability test, 1995 a constant need of new formability data for the TWBs to control the [2]. R.M. Hobbs & J.L. Duncan in: P.D. Harvey (Ed.), A course on failure of the formed parts. Saunders et. al. [27] advanced sheet metal Forming, ASM Metals Park, OH, 1979 conducted four different formability tests on the Tailor Welded [3]. Sheet Metal Forming Technology, Dr. NS Mahesh Blanks by using two base materials, first, 1.8 mm Aluminum- Killed [4]. Ruiquiang Zhang, Zhutao Shao, Jianguo Lin, A review on Drawing Quality (ASDQ) steel and second, 2.1 mm High Strength modelling techniques for formability prediction of sheet metal Low Alloy Steel (HSLA) and concluded that two modes of failures forming were occurring. Type 1 failure occurred when the principal stretch [5]. ISO 12004-2, Metallic material-sheet and strip- determination of axis lied parallel to the weld and fracture depended on the ductility. Forming Limit Curves. Part 2: Determination of forming limit Type 2 failure was the dominant forming failure which was not curves in laboratory, 2008, pp. 3-8 affected by welding operations. Their study further concluded that [6]. P.F. Bariani, S. Bruschi, A. Ghiotti, A. Tureta, Testing weld-line displacement is also an important parameter of formability Formability in the hot stamping of HSS, CIRP, Ann. 57(2008) in TWBs. 265-268 6. Future scope in formability analysis of sheet metal [7]. Z. Marciniak, K. Kuczynski, Limit strains in the process of forming stretch-forming sheet metal, Int. J. Mech. Sci. 9 (1967) 609-620 At the present stage, different FLC models have different advantages [8]. K. Raghvan, A simple technique to generate in-plane forming to offer and at the same time their inherent limitations also. So, the limit curves and selected applications, Metall. Mater. Trans. 26 future of FLCs would be to develop new models that can increase (1995) 2075-2084 their advantages and at the same time suppressing their limitations. [9]. Testing and modelling of material behavior and formability in As seen in the discussion of strain rates it is evident that formability sheet metal forming, S. Bruschi, T. Altan, D. Banabic, P.F. of metals improves with high velocities to a great extent. This has led Bariani, A. Brosius, J. Cao, A. Ghiotti, M. Khraisheh, M. to researchers focus on techniques of high velocity forming as seen Merklein, A.E. Tekkaya in processes like Explosive forming, Electromagnetic Forming, etc. [10]. Kikuma T. Nakazima K (1971), Effects of the deformation conditions and mechanical properties on stretch forming of steel

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Volume 8 Issue 2 (2020) 82-85 ISSN 2347-3258 International Journal of Advance Research and Innovation sheets. Transactions of the Iron and Steel Institute of Japan 11: 827-839 [11]. Marciniak Z., Kuczynski K., Pokota T. (1973) influence of plastic properties of a material on the forming limit diagram for sheet metal in tension. International Journal of Mechanical Science 15: 789-805 [12]. M. El Sherbiny an. El: “Thinning and residual stresses of sheet metal in the deep drawing process”: Materials and design 55 (2014) 869-879 [13]. Ayres R.A, Braier WG, Sajewski VF (1979) Evaluating the limit dome height as a new measure of press formability, Journal of Applied Metalworking 1(1): 41-49 [14]. Swift HW (1952) Plastic instability under plane stress, Journal of the Mechanics and Physics of Solids 1:1-18 [15]. Dorel Banabic, Frederic Barlat, Oana Cazacu, Toshihiko Kuwabara, Anisotropy & Formability [16]. Hill, R: A theory of the yielding and plastic flow of Anisotropic Metals, Proc. Roy. Soc. London A193 (1948) 281-297 [17]. Y.G. An, H. Vegter, S. Melzer, P. Romano Triguero, Evolution of the plastic anisotropy with straining and its implication on formability for sheet metals [18]. SS. Hecker, Metallurg. Eng. Quart., 14 (1974) 30 [19]. Hans Conrad, Effect of the strain-rate sensitivity of the flow stress on stretch formability of sheet metals. [20]. B.M. Dariani, G.H. Liaghat, M Gerdooei, Experimental investigation of sheet metal formability under various strain rates [21]. Hu. X and Daehn G.S., Effect of velocity on localization in uniaxial tension, Acta Mater., 1996, 44, 1996,1021 [22]. Gerdooei M and Dariani B.M., Dynamic analysis instability of sheet metal under biaxial stretching. Amirkabir J. Sci. Technol., 2007, 18 (67-B), 31-39 [23]. K. Schmidt, Growing sheet me stamping blanks, metal forming, (September 1994) ,43-46 [24]. E. Ahmed, U. Resgen, M. Schleser and O. Mokrov, on formability of Tailor laser welded blanks of DPI/RIP steel sheets [25]. Mustafa A. Ahmetoglu, Dirk Brouwers, Leonid Shulkin Laurent Taupin, Gary L. Kinzel, Taylan Altan, Deep drawing of round cups from tailor welded blanks [26]. Hisashi Kusuda, Toshiyuki Takasago, Fumiaki Natsumi, Formability of tailored welded blanks [27]. F.I. Saunders & R.H. Wagoner, Forming of Tailor welded blanks [28]. Emmens W, Van den Boogerd AH (2009) Cyclic stretch Bending: Mechanics stability and formability

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